1. Field of the Invention
The invention relates to a method for discriminating microseismic signals induced by the development of an underground reservoir, from among seismic signals emitted within the seismic monitoring operations.
2. Description of the Prior Art
In order to follow the evolution of an underground reservoir under development, it is well-known to permanently install seismic sources and seismic receivers, at the surface or in wells, whether for reservoir exploration or for development wells, and to carry out, at predetermined time intervals, recording of the seismic signals from the formation. These signals may be seismic signals induced by the site development activity or reflected by the geologic discontinuities of the formation in response to the emission of disturbances in the ground by means of one or more seismic sources. Comparison of the recordings performed at different times of the site development provides information on the evolution thereof.
Various long-term seismic monitoring systems are described for example in the following patents EP-591,037 corresponding to U.S. Pat. No. 5,461,594, French Patent 2,593,292 corresponding to U.S. Pat. No. 4,775,009, and French Patent 2,728,973 corresponding to U.S. Pat. No. 5,724,311) or French Patent 2,775,349.
EP Patent 748,457 corresponding to U.S. Pat. No. 5,724,311 filed by two of the applicants describes a permanent monitoring system allowing obtaining perfect reproducibility of the operating conditions in repetitive seismic monitoring operations carried out in an underground zone traversed by at least one well or borehole, notably in an underground gas storage reservoir. This system comprises, installed on a permanent basis, one or more set(s) of seismic receivers (buried at the surface or placed in one or more wells), several repetitive seismic sources (buried or at the surface), and a permanent linking network for selective energy supply to these sources. A central station remotely controls selectively each one of these sources and records the seismic signals coming from the underground zone in response to the seismic waves transmitted selectively in the ground by the sources.
All these permanently installed sources whose coupling with the surrounding formations remains stable and this supply network, at least partly buried and whose surface coverage area is limited, allows carrying out a series of long-term seismic monitoring operations under stable operating conditions, without any risk of incompatibility with the development site activities.
French Patent 2,775,349 corresponding to U.S. Pat. No. 6,182,012 filed by the same two applicants describes a method and a device intended for permanent monitoring of an underground formation in which cavities are formed for seismic sources (preferably deep enough to be coupled with the formation below the weathered zone (WZ)), at least one well is drilled either below each source or in the immediate vicinity thereof. An antenna having a plurality of seismic receivers (geophones, hydrophone, etc.) is lowered into each well thus drilled and coupled with the surrounding formation. With this device, emission-reception cycles are carried out with emission of seismic waves in the formation by at least one source and acquisition of the signals picked up by the seismic receivers, in response to the waves reflected by the formation. The receivers can for example be associated with a fluid transfer tube lowered in the well in order to connect an underground zone to a surface development device and coupled with the formation surrounding the well.
The various sources of the seismic monitoring device can be actuated successively with a sufficient time interval between the triggering times for reception of the waves reflected by the zone being investigated. It is also possible to use seismic sources emitting the same signals that are triggered simultaneously to increase the power emitted.
French Patent 2,805,051 filed by the applicants describes a method and a system for seismic monitoring of an underground formation. As diagrammatically shown in FIGS. 1 to 4, the system comprises for example a network of seismic antennas 2 each having a series of seismic pickups 4 arranged at regular intervals along a well 3 drilled in the ground. This network can be regular as shown in FIG. 2 or irregular. The pickups are, for example, vertically oriented one-directional or multi-axis (triphones) geophones and/or hydrophones. A seismic source 5 is arranged near to each antenna 2. Piezoelectric type vibrators such as those described in French Patent 2,791,780 U.S. Pat. No. 6,338,394 filed in the joint names of the applicants are advantageously used as sources and permanently installed in the immediate vicinity of each antenna 2. An electronic control and recording module 6 (FIG. 1) can be associated with each antenna. These electronic modules 6 can be connected to a central control and synchronization laboratory 8. According to another implementation mode diagrammatically shown in FIG. 4, antennas 2 are directly connected to a central laboratory 8 including all the individual electronic control and recording modules 6 (FIG. 1).
Seismic waves are emitted in the underground formation by one or more seismic sources (vibrators) 5. In the case of multiple sources, the latter emit simultaneously and are controlled by orthogonal signals so as to form a composite vibrational signal. The vibrators work permanently or at least for a sufficiently long time to obtain a sufficient signal-to-noise ratio. The seismic waves generated by seismic sources 5 are propagated downwards (downgoing waves 9). These incident waves are first recorded by receivers 4 of each well 3. The waves reflected by the discontinuities of the zone (seismic interfaces) are propagated upwards. These upgoing waves 10 are also recorded by the various receivers 4. The upgoing and downgoing waves are thus superimposed in the seismograms. The records are subjected to the conventional processings known in the art suited for those obtained by means of the VSP technique (Vertical Seismic Profiling). Finally, the respective contributions of the seismic sources to the composite vibrational signal are discriminated and the seismograms equivalent to those that would be obtained by actuating the seismic sources separately are reconstructed. Sinusoidal signals of different frequencies, in their fundamental components as well as in their respective harmonics, or signals based on wavelets, Legendre polynomials or random series, etc., are for example used as orthogonal signals.
In the case notably where the orthogonal signals emitted are sinusoids, discrimination of the respective contributions of the seismic sources is for example performed by determining the amplitude and the phase of the composite vibrational signal at the fundamental frequencies of the control signals applied to the seismic sources. This discrimination comprises for example weighting of the signals recorded by a bell weighting (or apodizing) factor and determination of the amplitude and of the phase of the composite signal, or selection by Fourier transform of lines of the complex spectrum associated respectively with the various weighted signals. Reconstruction of the seismograms corresponding specifically to the various seismic sources is carried out for example by applying, after separation thereof, an inverse Fourier transform to the lines associated respectively with the various weighted signals.
Furthermore, it is well-known that reservoir development induces mechanical stress variations in the reservoir and in its overlying bed. These stress variations have the effect of generating seismic signals that are propagated in the formations. The receivers of the various antennas 2 record these signals (upgoing waves 11).
Since seismic sources 5 are arranged much closer to receivers 4 of antennas 2 than to the reservoir and since the energy that is emitted is much more intense than the energy of the induced microseismic signals generated in the reservoir, the microseismic signals are generally concealed and undetectable.